Quantum‐Defect‐Minimized, Three‐Photon‐Pumped Ultralow‐Threshold Perovskite Excitonic Lasing.

Three‐photon‐pumped (3PP) excitonic lasing in inorganic semiconductor quantum dots (QDs) is of particular importance for near‐infrared biophotonics and optical communications. However, the implementation of such lasers has been hindered severely by the required high pump thresholds. Here, 3PP excitonic lasing of all‐inorganic cesium lead bromide perovskite QDs (CsPbBr3 PQDs) embedded in a whispering‐gallery microcavity is demonstrated, and achieving a record low threshold of 3 mJ cm−2 by tuning the 3P pump energy in resonance with the S exciton state. Wavelength‐dispersive Z‐scan spectroscopy reveals that such reduced lasing threshold is attributed to the exciton resonance enhanced multiphoton absorption, which, as disclosed by the kinetics analysis of transient absorption spectroscopy (TAS), leads to the appearance of net gain at a pump fluence as low as 2.2 mJ cm−2, corresponding to an average S exciton population of 1.5. A microscopic model incorporating the quantum master equation reproduces the TAS results and provides the intrinsic parameters of biexciton relaxation for lasing. The 3PP resonant excitonic transition is the most favored multiphoton pumping process that minimizes quantum defect (6.8% of the pump photon energy) to realize optical gain at low threshold, marking a major step toward using all‐inorganic perovskite QDs for on‐chip integrated microlasers and multiphoton bioimaging. [ABSTRACT FROM AUTHOR]